FIELD OF THE INVENTION
The present invention relates to implantable medical devices and methods, and more particularly, to the use of hysteresis within a rate-responsive pacemaker.
A pacemaker is an electronic device, usually implanted, that generates electrical stimulation pulses that are delivered to the heart of a patient in order to cause the patient's heart to contract, or beat, at a prescribed rate. For example, if it is desired that the heart rate of the patient be maintained at 70 beats per minute (bpm), then the pacemaker is set to generate stimulation pulses at a rate of 70 pulses per minute (ppm), with each stimulation pulse, causing the heart to beat (i.e., causing the cardiac muscle tissue to depolarize and contract).
Most modern pacemakers are programmable. That is, the rate at which stimulation pulses are generated, as well as numerous other operating parameters associated with the pacemaker, are parameters that may be selected, typically using noninvasive programming means by the patient's physician or other medical personnel. The operation and programmable features of modern implantable pacemakers are well known and described in the art. See, e.g., Furman et al. A Practice of Cardiac Pacing, Futura Publishing Company, Inc. (Mount Kisco, N.Y. 1986); Moses et al.; A Practical Guide to Cardiac Pacing, Little Brown and Company (Boston/Toronto 1983). See also, e.g., U.S. Pat. Nos. 4,712,555 (Thornander et al.); 4,788,980 (Mann et al.); 4,940,052 (Mann et al.); and 4,944,298 (Sholder), which patents are incorporated herein by reference.
In order to allow the patient's heart to beat "on its own," without the need for an external stimulation pulse generated by a pacemaker, it is common in the art to operate an implantable pacemaker in a demand mode of operation. In a demand mode, the pacemaker monitors the heart to determine if a cardiac contraction (heartbeat) has naturally occurred. Such natural (non-stimulated) contractions, also referred to as "intrinsic" or "sinus" cardiac activity, are manifest by the occurrence of recognizable electrical signals that accompany the depolarization or contraction of cardiac muscle tissue. (While the depolarization of cardiac muscle tissue is technically a separate event from the actual contraction of cardiac muscle tissue --with depolarization immediately preceding contraction--for most purposes, and certainly for purposes of the present application, depolarization and contraction may be considered as simultaneous events, and the terms "depolarization" and "contraction" are used herein as synonyms.) The depolarization of atrial muscle tissue, for example, is manifest by the occurrence of a signal known as the "P-wave." Similarly, the depolarization of ventricular muscle tissue is manifest by the occurrence of a signal known as the "R-wave." The sensing of signals representing the occurrence of P-waves and R-waves, and other related signals, comprise the electrocardiogram (ECG) of the patient when sensed external to the heart, e.g., at the skin. When these same signals are sensed internal to or on the heart, they are generally referred to as the electrogram (EGM) of the heart.
In a demand mode of operation, the pacemaker monitors the heart for the occurrence of P-waves and/or R-waves. If such signals are sensed within a prescribed time period or time window, typically referred to as an "escape interval," then the escape interval is reset (i.e., restarted) and no stimulation pulse is generated. The escape interval is measured from the last heartbeat, i.e., from the last occurrence of a P-wave (if the atrium is monitored), or R-wave (if the ventricle is monitored), or the generation of a stimulation pulse (if natural activity does not occur). If the escape interval "times-out," i.e., if a time period equal to the escape interval has elapsed without the sensing of a P-wave and/or R-wave (depending upon which chamber of the heart is being monitored), then a stimulation pulse is generated at the conclusion of the escape interval, and the escape interval is reset, i.e., restarted. In this way, the pacemaker provides stimulation pulses "on demand," i.e., only as needed, when intrinsic cardiac activity does not occur within the prescribed escape interval.
Hence, it is seen that it is the escape interval that defines the rate at which stimulation pulses are generated by the pacemaker. In a demand mode of operation, the pacemaker provides stimulation pulses, when needed, at the rate set by the escape interval, even through the natural or intrinsic rhythm of the heart may occur at a rate faster than the escape interval. For example, if it is desired that the heart rate never slow to a rate less than 60 bpm, then the escape interval is set to 1000 milliseconds (msec), or 1 second, corresponding to the period of a 1 Hz signal. So long as intrinsic cardiac activity occurs at a rate faster than 60 bpm, i.e., so long as less than 1000 msec occurs between natural heartbeats, then no stimulation pulse is generated. However, as soon as 1000 msec occurs since the last natural heartbeat, a stimulation pulse is generated. In this way, the heart is assured of beating at least once every 1000 msec.
One of the programmable modes that has been used with programmable pacemakers for many years is a mode known as the "hysteresis" mode. The hysteresis mode is used in conjunction with selected other modes, such as single-chamber demand pacing, to allow the natural sinus rhythm of the heart to persist at rates less than the programmed minimum rate of the pacemaker. The programmed minimum rate of the pacemaker, in turn, sets the pacemaker escape interval. During pacing, i.e., during those times when the pacemaker is generating stimulation pulses, the pacemaker thus stimulates the heart at the rate set by the escape interval, i.e., upon the timing-out of each escape interval. When the hysteresis mode is enabled, sensed cardiac activity causes the pacemaker escape interval to be extended, or lengthened, thereby providing a longer period of time within which natural cardiac activity may occur before the pacemaker steps in to generate a stimulation pulse. Should the intrinsic rate of the heart fall below the programmed hysteresis rate, i.e., should no intrinsic cardiac activity be sensed during the lengthened escape interval, then a stimulation pulse is generated, and the escape interval reverts back to its initial value, as determined by the programmed minimum rate.
As an example of the hysteresis mode of operation, assume that the programmed minimum rate of a pacemaker is 70 ppm, corresponding to an escape interval of 857 msec. Further, assume that the programmed hysteresis rate is 50 ppm, corresponding to an extended escape interval of 1200 msec. At these values, the pacemaker maintains a minimum heart rate of 70 bpm, providing a stimulation pulse every 857 msec. Upon the occurrence of intrinsic cardiac activity, the escape interval is extended 350 msec, making the total extended escape interval equal to 1200 msec. So long as intrinsic cardiac activity continues to be sensed during this extended escape interval, i.e., so long as the natural sinus rhythm does not slow to a rate less than 50 bpm, then no stimulation pulses are generated. However, as soon as the extended escape interval times-out without the occurrence of natural cardiac activity, i.e., as soon as the natural sinus rhythm drops to a rate less than 50 bpm, then a stimulation pulse is generated and the escape interval is shortened back to its original value, i.e., back to 857 msec. The next stimulation pulse, and all subsequent stimulation pulses, occur at the programmed minimum rate, 70 ppm, unless interrupted by a sensed natural cardiac activity as described above.
The hysteresis mode of operation advantageously serves a dual purpose. First, it allows the patient's heart to beat at its own rhythm more often. Second, it prolongs pacemaker longevity. Pacemaker longevity is determined by the limited energy stored within the pacemaker. Such energy is stored in a battery. Most of the power drained from the battery is caused by the generation of stimulation pulses. Hence, the frequency with which stimulation pulses are generated has a profound effect upon pacemaker longevity. Advantageously, the hysteresis mode reduces the frequency at which stimulation pulses must be generated for most patients. Hence, the longevity of the pacemaker is increased when a hysteresis mode is invoked.
Disadvantageously, the hysteresis mode is not typically available for certain pacemaker modes of operation. For example, in dual-chamber pacing, such as operation in a DDD mode of operation (see, e.g., U.S. Pat. No. 4,944,298 (Sholder) for an explanation of the various pacemaker modes, and the three- or four-letter code used to identify such modes) hysteresis has not generally been used. This is because the whole idea behind DDD pacing is to allow the heart to be paced at a rate set by the sino-atrial (SA) node, thereby providing a true physiologic pacing regime with the maintenance of AV synchrony.
In atrial-based DDD pacing, all timing is controlled from the sensing of atrial activity (a P-wave). When a P-wave is sensed, two separate timers are started that operate in parallel. A first timer defines an atrial escape interval which, if timed-out, results in an atrial paced event. A second timer defines a separate AV delay which, if timed-out, results in a ventricular paced event. The first and second timers both start upon sensing atrial activity. The AV delay timer does not affect the basic atrial escape interval timer. The atrial escape interval timer thus controls the basic functioning rate of the pacemaker from atrial to atrial event. The ventricle is paced, if needed, at a rate that tracks the sensed atrial rate. If no atrial activity is sensed, then the atrium is also paced at a rate equal to the minimum set rate. Any reduction in the atrial paced rate below the minimum rate, such as might be introduced by hysteresis, has heretofore been viewed as not necessary, and has not been pursued. Nonetheless, even when operating in such atrial-based mode, there still remains a need to enhance pacemaker longevity, as well as a need to allow the heart to beat at its own rhythm more often. Such needs could best be met by providing a hysteresis mode of operation for atrial-based DDD pacing.
In addition to above-described atrial-based DDD pacing, there is also a second type of DDD operation known as ventricular-based DDD pacing. In ventricular-based DDD pacing (sometimes referred to as ventricular-based timing), two sequential timers are used. After an atrial event, an AV Delay timer is started. There is no coincident starting of a basic escape interval timer as occurs in the atrial-based DDD pacing. If the AV Delay timer times-out all the way, a ventricular pulse (V-pulse) is provided. If an R-wave is sensed before the AV Delay timer times-out, such sensing terminates the AV Delay timer. The sensing of an R-wave or the generating of a V-pulse thus comprise a ventricular event. After a ventricular event, an Atrial Escape Interval timer begins. If this Atrial Escape Interval timer times-out all the way, an atrial pulse (A-pulse) is provided. If a P-wave is sensed before the Atrial Escape Interval timer times-out, such sensing terminates the Atrial Escape Interval Timer. The sensing of a P-wave or the generating of an A-pulse thus comprise an atrial event, which atrial event again starts the AV Delay timer. Thus, in this way, the two timers of a ventricular-based DDD pacing system, function sequentially as opposed to the concurrent or parallel function of the timers in the atrial-based DDD pacing system. However, as with the atrial-based DDD pacing system, a ventricular-based DDD pacing system also faces a need to enhance pacemaker longevity, as well as a need to allow the heart to beat at its own rhythm more often. Such needs could best be met by providing a hysteresis mode of operation for ventricular-based DDD pacing.
Further, the hysteresis mode has not heretofore been used with rate-responsive pacing. In a rate-responsive pacer (note that the terms "pacer" and "pacemaker" are used herein as synonyms), some sort of physiological sensor is used to sense and measure a physiological parameter indicative of how fast the heart should be paced by the pacemaker (the "sensor-indicated rate"). For example, during periods of high physical activity, e.g., exercise, it is desirable that the heart be paced at a faster rate. To this end, a rate-responsive pacer senses such physical activity and adjusts the escape interval of the pacer accordingly so that the pacer's stimulation pulses are provided on demand at a faster rate during periods of sensed high physical activity (or other sensed physiological indicators). Hysteresis has not heretofore been used with rate-responsive pacers because the escape interval of such pacers is already changing as a function of the sensor-indicated rate, and any variations of the escape interval beyond those indicated by the pacer's physiological sensor have been viewed as undesirable and disruptive to the sensed physiologic pacing rate. Nonetheless, even when operating in a rate-responsive mode, there still remains a need to enhance pacemaker longevity, as well as a need to allow the heart to beat at its own rhythm more often. There is thus a need in the art for some type of hysteresis mode that can be used with a rate-responsive pacer.